This invention pertains to methods and apparatuses for the capacitive radio frequency (RF) dielectric heating of food and biological products.
A variety of different methods are available for the thermal processing of various materials. Heat is supplied by hot water, steam, resistive heating elements, burners, torches, ovens, electrical conduction (ohmic heating), induction heating (magnetic), capacitive heating (dielectric), and electromagnetic radiative heating (resonant ovens, cavities or chambers) and many other heating methods. Applications include sterilization, pasteurization, thawing, melting, curing, drying, bonding (e.g., laminates), welding, brazing, heating for chemical reactions, and many others. Heated materials include ceramics, rubber, plastics (and other polymers), composites, metals, soils, wood and many types of biological materials including food.
An important application of heating technologies is in the area of the pasteurization and sterilization of foods, particularly foods in large-dimensioned packages. Food safety and quality is becoming an increasingly important topic with the many incidents where people have become sick or died due to unkilled microbial populations in food. For example, alfalfa and radish seeds are raw agricultural commodities that can become contaminated with organic material that harbor pathogens such as Salmonella or E. coli O157:H7 during growing and harvest. Seed processing and storage procedures are aimed at reducing varietal contamination of seeds through the elimination of weed seeds and foreign matter. Such seed cleaning and certification programs insure varietal purity, but provide no means of food safety intervention for seeds destined for sprouting and consumption as food. As a result, there are increasing reports of microbial outbreaks in sprouted seed products such as radish and alfalfa sprouts. Human salmonellosis (due to Salmonella bacteria) and outbreaks of E. coli O157:H7 have been associated with the consumption of alfalfa and radish sprouts in several countries. Alfalfa and radish sprouts, a definitive highly nutritious and perceived healthy food, have been implicated in multi-site outbreaks of food-borne illnesses. Seeds were linked to about 150 confirmed cases of salmonellosis in Oregon and British Columbia in 1996. Also in 1996, radish sprouts were associated with Japan""s largest recorded outbreak of E. coli O157:H7 infection with an estimated 11,000 cases that led to eleven deaths. In June and July 1997, simultaneous outbreaks of E. coli O157:H7 infections in Michigan and Virginia were independently associated with eating alfalfa sprouts grown from the same seed lot. A total of 60 people with E. coli O157:H7 infection were reported to the Michigan Department of Community Health and 48 cases reported to the Virginia Department of Health. Recently, the California Department of Health Services identified six cases of E. coli O157:NM with illness onsets from Jun. 16 through Jun. 27, 1998, caused by eating an alfalfa-clover sprout mixture.
The lack of standardization in some heating time/temperature relationships that are required to ensure food product safety is also attracting more focus. In addition, food quality or taste/texture issues are important in our selective consumer oriented society. Therefore there is a need for a heating technology that will achieve the desired microbial kill rates uniformly over that whole food product in a reasonable amount of time with a minimum altering of the overall quality of the food.
In the seafood industry, for example, existing heating technologies for the pasteurization of seafoods employ either hot water or steam. These technologies have several limitations including reliance on thermal conduction from the product surface (resulting in non-uniform heating), slow heating rates (especially in the product center), large floor space requirements, poor overall energy efficiency, generation of large amounts of waste water and limitations on the product geometry (i.e., need to be thin or flat).
Capacitive radio frequency (RF) dielectric heating is used in several industries. They include the drying of various wood and sawdust products in the timber industry, preheating and final drying of paper, drying of textiles, drying of glass fibers and spools, drying water-based glues in the paper-cardboard industry, drying pharmaceutical products, welding plastics, sealing, preheating plastics prior to forming, firing foundry cores in casting, polymerization of fiber panels, gluing of woods such as laminated plywood, printing and marking in the textile, leatherware and shoe industries, melting honey, heating rubber prior to vulcanization, welding glass formed sections, bonding multi-layer glass products, drying of powders, drying leathers and hides, curing of epoxy, curing of plastisol, curing of brake linings, impregnating resins, thermosetting adhesives, curing hardboard and particle board, and many other applications.
The use of capacitive (RF) dielectric heating methods for the pasteurization and sterilization of foods offer several advantages over non-electromagnetic heating methods. These include rapid heating, near independence of the thermal conductivity of the medium (i.e., heat internal portions of medium directly), high energy efficiency, good heating even in the absence of DC electrical conductivity, high energy densities, reduced production floor space, and easy adaptation to automated production batch and/or continuous flow processing. Because capacitive (RF) dielectric heating is rapid, the food product being heated loses less moisture than in conventional heating processes, which is advantageous.
Another application of this technology is in the thawing of frozen foods. Common thawing applications again rely on the thermal conduction of heat from the surface to the interior to provide thawing. Due to freshness and product quality constraints thawing often is done by immersion in water baths that are only slightly above freezing themselves or in refrigerators set to slightly above freezing (e.g., 35-40xc2x0 F.). Thawing times are often very long. With capacitive heating technologies that heat over the entire volume uniformly, thawing can be performed much more rapidly.
Capacitive (RF) dielectric heating differs from higher frequency electromagnetic radiative dielectric heating (e.g., microwave ovens) in that with capacitive heating the wavelength of the chosen frequency is large compared to the dimensions of the sample being heated whereas with electromagnetic radiative heating the wavelength is comparable or even small compared to the dimensions of the sample being heated. An example of capacitive heating is two large parallel electrodes placed on opposite sides of a wood sample with an AC displacement current flowing through it to heat and dry the wood. An example of electromagnetic radiative heating is a metal chamber with resonant electromagnetic standing wave modes such as a microwave oven. Capacitive heating also differs from lower frequency ohmic heating in that capacitive heating depends on dielectric losses and ohmic heating relies on direct ohmic conduction losses in a medium and requires the electrodes to contact the medium directly (i.e., cannot penetrate a plastic package or air gap). (In some applications, capacitive and ohmic heating are used together.)
Capacitive (RF) dielectric heating methods offer advantages over other electromagnetic heating methods. For example, capacitive (RF) dielectric heating methods offer more uniform heating over the sample geometry than higher frequency radiative dielectric heating methods (e.g., microwave ovens) due to superior or deeper wave penetration into the sample as well as simple uniform field patterns (as opposed to the complex non-uniform standing wave patterns in a microwave oven). In addition capacitive (RF) dielectric heating methods operate at frequencies low enough to use standard power grid tubes that are both lower cost (for a given power level) as well as allow for generally much higher power generation levels than microwave tubes.
Capacitive (RF) dielectric heating methods also offer advantages over low frequency ohmic heating. These include the ability to heat a medium that is enclosed inside an insulating plastic package and perhaps surrounded by an air or de-ionized water barrier (i.e., the electrodes do not have to contact the media directly). The performance of capacitive heating is therefore also less dependent on the product making a smooth contact with the electrodes. Capacitive (RF) dielectric heating methods are not dependent on the presence of DC electrical conductivity and can heat insulators as long as they contain polar dielectric molecules that can partially rotate and create dielectric losses. A typical existing design for a capacitive dielectric heating system is described in Orfeuil, M. 1987. Electric Process Heating: Technologies/Equipment/Applications. Columbus: Battelle Press.
Capacitive (RF) heating devices have been used in the food industry, but the reported energy efficiency has been low and heating has not always been uniform. Proctor Strayfield has developed a magnatube pasteurization system (Koral, A. L., 1990. Proctor-Strayfield Magnatube Radio Frequency Tube Heating System. Proctor Strayfield, A Division of Proctor and Schwartz, Inc.) that has been demonstrated to be successful in the cooking/sterilization of scrambled eggs as well as in the creation of a xe2x80x9cskinlessxe2x80x9d meatloaf from a pumped slurry using a vertical tube system. Houben et. al of the Netherlands (Houben, J., Schoenmakers, L., van Putten, E., van Roon, P. and Krol, B. 1991. Radio-frequency pasteurization of sausage emulsions as a continuous process. J. Microwave Power and Electromagnetic Energy. 26(4): 202-205.) in 1991 showed that sausage emulsions could be successfully pasteurized using RF heating. Bengtsson et al of Sweden (Bengtsson, N. E., and W. Green. 1970. Radio-Frequency Pasteurization of Cured Hams. Journal of Food Science. V35: 681-687) in 1970 demonstrated that cured hams could be pasteurized successfully by RF heating. RF heating feasibility experiments were conducted on packaged and unpackaged surimi seafood samples at a test facility of PSC, Inc. of Cleveland, Ohio (Kolbe, E. 1996. Heating of packaged surimi seafoods in a commercial RF oven. Unpublished information. OSU Dept. of Bioresource Engineering.). The test system was a high power single-frequency capacitive heater set at 18 MHz. Tests on samples placed between parallel electrodes showed that when properly oriented, surimi seafoods could be heated to pasteurization temperatures (85xc2x0 C.) in less than 10 minutes. The results also showed, however, that packaging can be a complicating factor. For example, small amounts of food trapped in the packaging seams can cause rapid local heating and burning.
Some prior work in the area of dielectric heating has been conducted on seed germination enhancement effects. The possibility of utilizing dielectric energy for stimulating or improving the germination and growth of seeds and for controlling insects has been variously considered for the last forty years (Nelson, S. O. and Walker, E. R. 1961, xe2x80x9cEffects of radio-frequency electrical seed treatment,xe2x80x9d Agricul. Eng. 42(12): 688-691; Nelson, S. O. 1976, xe2x80x9cUse of microwave and lower power frequency RF energy for improving alfalfa seed germination,xe2x80x9d J. Microwave Power 12(1):67-72; Nelson, S. O. 1996, xe2x80x9cReview and assessment of radio-frequency and microwave energy for stored-grain insect control,xe2x80x9d Transactions of the ASAE. 39(4):1475-1484.). Reported effects ranged from accelerated germination and early growth and the killing of fusarium spores to early flowering and high yields of plants grown from treated seeds. Nelson and Walker (1961) reported that brief exposure of alfalfa containing considerable hard seed shells to electrical fields has been highly successful in reducing the hard-seed percentages and producing a corresponding increase in normal seedling germination. Also, benefits from electric treatment have been shown in alfalfa seeds for up to 21 years in storage with no evidence of any short or long term detrimental effects (Nelson, 1961, 1976). Nelson (1976) found that the moisture content of seeds at the time of treatment influenced the degree of response. Generally seeds of lower moisture content responded more favorably to treatment than high moisture content seeds. The final temperature of seeds treated at any given moisture content seemed to be a good indicator of the degree of favorable response. Some work has been done using higher frequency microwave heating for the treatment of seeds. Cavalcante et al. (Cavalcante, M. J. B. and Muchovej, J. J. 1993, xe2x80x9cMicrowave irradiation of seeds and selected fungal spores,xe2x80x9d Seed Sci. and Technol. 21:247-253) investigated the use of microwave irradiation on seeds and its effects on the control of selected fungal spores.
Some work has been done to characterize the dielectric properties of food and packaging materials. There is preliminary data at lower frequencies for polymers to show temperature-dependent Debye resonance effects (Malik, T. M., R. E. Prud""Homme. 1984. Dielectric Properties of Poly(xcex1-Methyl-xcex1-N-Propyl-xcex2-Propiolactone)/Poly(Vinyl Chloride) Blends. Polymer Engineering and Science. v24, n2 p 144-152; Scarpa, P. C. N., Svatik, A. and Das-Gupta, D. K. 1996. Dielectric spectroscopy of polyethylene in the frequency range of 10xe2x88x925 Hz to 106 Hz. Polymer Eng. and Sci. 36(8): 1072-1080). And, for food in the medium frequency ranges, limited tabulated data exists (Von Hippel, A. R., 1954. Dielectric Materials and Applications. MIT Press; Kent, M. 1987. Electrical and Dielectric properties of food materials. Science and Technology Publishers, England; Mudgett, R. E. 1985. Electrical Properties of Foods. In Microwaves in the Food Processing Industry, R. V. Decareau (Ed.). New York: Academic Press; Pethig, R. 1979. Dielectric and Electronic Properties of Biological Materials. Chichester: John Wiley and Sons, Inc.; Tinga, W. R. and S. O. Nelson. 1973. Dielectric Properties of Materials for Microwave Processing-Tabulated. J. of Microwave Power. 8:1-65; Tran, V. N. and Stuchly, S. S. 1987. Dielectric properties of beef, beef liver, chicken and salmon at frequencies from 100 to 2500 MHz. J. Microwave Power. 29-33). Most data for food has been collected at higher frequencies ( greater than 100 MHz) and tied closely to the dielectric behavior of the water in the medium, for applications toward microwave ovens.
A specific disadvantage of capacitive (RF) dielectric heating methods is the potential for thermal runaway or hot spots in a heterogeneous medium since the dielectric losses are often strong functions of temperature (e.g., small pockets of a lossy dielectric food material, for example a small thermal mass trapped in the seams of a package, may heat rapidly and could burn itself and melt the package). Another disadvantage of capacitive heating is the potential for dielectric breakdown (arcing) if the electric field strengths are too high across the sample (making sample thicker and reducing air gaps allows operation at a lower voltage).
The use of edible films to extend shelf life of food products and protect them from harmful environmental effects has been emphasized in the recent years. Interests and research activities in edible films have been especially intense over the past ten years. Edible films are very promising systems for the future improvement of food quality and preservation during processes and storage. Indeed, edible films can be used where plastic packaging cannot be applied. For example, they can separate several compartments within a food. Although edible films are not meant to totally replace synthetic films, they do have the potential to reduce packaging and to limit moisture, aroma, and lipid migration between food components where traditional packaging cannot function.
An edible film is defined as a thin layer of one or more edible materials formed on a food as a coating or placed (preformed) on or between food components. Most edible films are natural polymers obtained from agricultural products such as animal and vegetable proteins, gums, and lipid and are perfectly biodegradable and usually water soluble. The general materials that are used to manufacture edible films are cellulose ethers, starch, corn zein, wheat gluten, soy proteins and milk protein. Examples include methyl cellulose (MC), hydroxypropyl cellulose (HPC), sodium and calcium caseinates (SC or CC), and whey protein concentrates (WPC).
The performance of edible packaging is comparable to that of traditional synthetic polymer films with respect to mechanical strength, barrier properties, and compatibility. Applications of edible packaging include its use in inhibiting migration of moisture, oxygen, carbon dioxide, aromas, and lipids, etc. within composite foods; carrying food ingredients (e.g., antioxidants, antimicrobials, flavor); and/or improving mechanical integrity or handling characteristics of the foods.
Moisture transport through polymer films is influenced by several polymer properties including chemical structure, method of polymer preparation, polymer processing condition, free volume, density, crystallinity, polarity, tacticity, crosslinking and grafting, orientation, presence of additives, and use of polymer blends. An increase in crystallinity, density, orientation, molecular weight or crosslinking results in decreased permeability of edible films.
Although capacitive (RF) dielectric heating systems have been used for heating foods in the past, there remains a need for improved methods and apparatuses to rapidly, efficiently and uniformly heat food products or parts of food products.
It has now been discovered that certain capacitive (RF) dielectric heating devices and/or methods can be used to rapidly, efficiently and/or uniformly heat food products, including conventional foods and seeds, as well as any related packaging, for safe pasteurization, sterilization and/or thawing.